Using the new system, the average woman is estimated to have drunk 9.4 units a week in 2005 - a 45% increase on the old figure from 6.5 units a week.

For men, the new figure is 19.9 units a week - a 26% increase on the old figure of 15.8 units a week.

Men are advised to drink no more than 21 units a week, and women no more than 14.

The report warns that the strength of wine has increased sharply in recent years, with most table wines now 11.5% to 13.5% alcohol by volume (ABV).

This has been mirrored by many types of lager and beer.

In addition, research suggests the average size of a wine glass used in pubs and bars is now 169ml, compared to the 125ml measure previously used in the calculations.

Old assumptions

When drinking surveys were first carried out in the 1970s it was assumed that a glass of table wine, a single measure of spirits or half a pint of beer equalled one unit of alcohol.

Now the ONS will assume that a glass of wine equals two units, while a large can of strong beer (6.5%) will count as three units.

An Alcohol Concern spokesman said the government and the drinks industry had to work harder to improve consumer awareness.

"There's always been big gap between how much people think they drink and how much they actually drink, but with changes in alcoholic strength this has clearly gotten worse.

"Current confusion may mean that large numbers of people are drinking at harmful or hazardous levels and aren't aware that they are putting themselves at risk in the long run."

Dr Vivienne Nathanson, of the British Medical Association, said: "This doesn't come as a surprise. Poor labelling, variations in glass size, and rising alcoholic percentages - especially in wine - all make it hard for people to know how much they're drinking, and to control it."

Public Health Minister Dawn Primarolo said the government was committed to tackling the problem of excessive alcohol consumption.

"The government is planning a major new multi-million pound campaign in the spring to coincide with the introduction of new labelling on all bottles and cans that will show the government's sensible drinking message and the alcohol unit content."